'solid-colour' is a Modern English compound formed from the adjective 'solid' and the noun 'colour'. 'Solid' ultimately comes from Latin 'solidus' meaning 'firm, dense', and 'colour' comes from Latin 'color' (via Old French 'colour').

'colour' entered English from Old French 'colour', from Latin 'color'. 'solid' comes from Latin 'solidus' through Late Latin/Old French into Middle English; the compound 'solid colour' developed in Modern English usage to describe a uniform hue and later became used in hyphenated forms like 'solid-colour' or 'solid-coloured'.

Initially, 'solid' referred to firmness/density and 'colour' to appearance or hue; combined, the phrase came to mean 'a hue that is uniform and without pattern', which is its current sense.